In a convolutional encoded feed forward error correction transmitter, a binary bit stream is divided into first and second bit streams respectively including sequential bits P.sub.1 (0), P.sub.1 (1), P.sub.1 (2), P.sub.1 (3), P.sub.1 (4) etc. and P.sub.2 (0), P.sub.2 (1), P.sub.2 (2), P.sub.2 (3), P.sub.2 (4) etc. In one-half rate convolutional encoding, the first and second bit streams are formed by combining adjacent bits in the original bit stream in accordance with a modulo 2 function, i.e., by using half adders responsive to the adjacent bits.
Because of the redundancy in the first and second bit streams, it is possible to remove some of the bits from these bit streams without substantial loss of information; such removal of bits from the first and second bit streams is generally referred to in the art as puncturing. Optimum puncturing codes for these bit streams are disclosed by Yasuda et al., "Development of Variable-Rate Viterbi Decoder and its Performance Characteristics," 6th International Conference on Digital Satellite Communications, Phoenix, Arizona, September 1983. Yasuda et al. discloses optimal puncturing rates from 2/3 to 16/17. The 2/3 puncturing code or rate is represented by:
TABLE I ______________________________________ 10 11 ______________________________________
Lines 1 and 2 of Table I respectively indicate puncturing operations performed on the bits of the first and second bit streams. The first place in line 1 indicates the puncturing operations to be performed on bits P.sub.1 (0), P.sub.1 (2), P.sub.1 (4) etc. of the first bit stream; the second place in line 1 indicates puncturing operations performed on bits P.sub.1 (1), P.sub.1 (3), P.sub.1 (5) etc. of the first bit stream; the first place in line 2 of Table I indicates the puncturing operations performed on bits P.sub.2 (0), P.sub.2 (2), P.sub.2 (4) etc. of the second bit stream; the second place in line 2 indicates the operations on bits P.sub.2 (1), P.sub.2 (3), P.sub.2 (5) etc. Values of 1 and 0 in Table I respectively indicate there is no puncturing and there is puncturing. The puncturing code of Table I is applied to the first and second bit streams to provide punctured bit streams:
TABLE II ______________________________________ P.sub.1 = P.sub.1 (0), P.sub.1 (2), P.sub.1 (4), P.sub.1 (6), P.sub.1 (8) etc. P.sub.2 = P.sub.2 (0), P.sub.2 (l), P.sub.2 (4), P.sub.2 (3), P.sub.2 (4), etc. ______________________________________
Thus bits P.sub.1 (1), P.sub.1 (3), P.sub.1 (5), P.sub.1 (7) etc. have been removed, i.e., punctured from the second bit stream. Yasuda et al. indicates the bit streams of Table II can be combined into a single serial bit stream by using a first in first out (FIFO) register such that the output of the first in first out register is:
TABLE III ______________________________________ P.sub.1 (0), P.sub.2 (0), P.sub.2 (1), P.sub.1 (2), P.sub.2 (2), P.sub.2 (3), P.sub.1 (4), P.sub.2 (4), P.sub.2 (5), P.sub.1 (6), P.sub.2 (6), P.sub.2 (7), P.sub.1 (8) etc. ______________________________________
The thus formed serial bit stream is applied to a modulator. Presumably, the serial bit stream applied to the modulator is a replica of the output of the FIFO, causing the modulator to emit a dual frequency shift key or bi-phase shift key signal. However, most satellite communications systems for binary data use a pair of orthogonal channels, generally referred to in the art as I and Q channels. Yasuda et al. is completely silent as to how the serial signal derived by the FIFO register can be divided into I and Q channels. Further, Yasuda et al. fails to disclose any receiving apparatus for the punctured convolutional encoded signal.
We are aware of a prior art two channel (I and Q) system employing punctured convolutional feed forward error correction techniques for handling only two specific punctured codes, viz: 3/4 and 7/8. In this prior art system, the convolutional encoded signals are punctured directly, i.e., no serial bit stream is formed, as disclosed by Yasuda et al. Hence, this prior art two channel transmitter and receiver system is dedicated to only two punctured codes and cannot be used for all the optimum punctured rates or codes disclosed by Yasuda et al.
It is, accordingly, an object of the present invention to provide a new and improved two channel punctured convolutional encoded transmitter, receiver and transmission method capable of handling all of the optimum punctured codes.
In the near future, a system is to be introduced wherein digitally encoded intelligence signals (particularly television programs) are to be transmitted from a terrestrial site via a geosynchronous satellite to receiver sites having antenna dishes with diameters no greater than approximately one meter feeding home television receivers. Two systems are currently envisaged, respectively employing terrestrial receiving antenna dishes having diameters of approximately 60 and 90 centimeters.
When the system using the 60 centimeter dishes is initially employed and for some time thereafter, a rate 1/2 convolutional encoded signal having a 2/3 punctured code is to be radiated from the geosynchronous satellite at a power level of 10 watts to the receiver antennas. After the initial phase-in period, the radiated power is to be increased 3 db, to 20 watts. It was initially thought that a punctured code of 7/8ths could be used for the higher power level. As a result of bit error ratio analyses we have performed, we have realized that the 7/8 punctured code is not acceptable and the 6/7 punctured code must be used to achieve acceptable results at the higher power level.
It is, accordingly, another object of the invention to provide a new and improved feed forward error correction transmitting method and apparatus utilizing plural punctured codes and plural power levels.
Another object of the invention is to provide a new and improved punctured encoding method and apparatus particularly adapted for transmission of intelligence signals (particularly encoded television program signals) through a geosynchronous satellite to terrestrial ground sites having antenna reflecting dishes with diameters no greater than approximately one meter.